The invention concerns a device for adjusting optical elements, in particular, for X-ray analysis, comprising a holding device for receiving the optical element, and at least two adjusting units, wherein each adjusting unit comprises one plunger. The invention also concerns a method for adjusting an optical element.
A device of this type is disclosed e.g. in the manual “Confocal Max-Flux Optics™” of the company Osmic Inc.
Optical elements, in particular X-ray mirrors, are used in X-ray analysis for monochromatisation, alignment or bundling of X-rays along the path from the X-ray source to the sample and from the sample to the detector.
Multi-layer X-ray mirrors have been used in the optical radiation path for some years in laboratory X-ray analysis devices such as e.g. X-ray diffractometers, and considerably increase the efficiency of the X-ray instruments. Mirrors of this type are disclosed e.g. in DE 198 33 524 and U.S. Pat. No. 6,041,099. These mirrors are mostly parabolic or elliptical and convert the divergent X-rays from laboratory X-ray sources into a parallel or focussed beam. In order to increase the reflectivity, the mirrors have multi-layers that can be produced through coating methods. In order to obtain optimum reflectivity, the thickness of the multi-layers must exhibit a specific functional dependence along the mirror. In X-ray mirrors as disclosed in DE 44 07 278, the layer periods (sum of two individual layers) must e.g. increase from approximately 4 nm to approximately 5 nm from the end of the mirror close to the source towards the end remote from the source. When typical X-ray sources are used with characteristic X-ray energies of between 5 keV and 20 keV, one obtains Bragg angles θ of typically 0.4 to 2 degrees in correspondence with the Bragg relationship (λ=wavelength, d=layer period)λ=2dsin θ  (1)
When multi-layer mirrors are used, the Bragg equation (1) is only an approximation. DE 198 33 524 gives a formula which is more exact than (1). In accordance therewith, the mirrors are operated at small angles of incidence, i.e. under grazing incidence angles. Depending on the selected materials of the multi-layers, the Bragg peaks have widths in the range between 0.25 and 2 mrad, i.e. in the range of a few hundredth degrees (full width at half maximum, i.e. a variation of this value produces a 50% loss in intensity). The X-ray sources used have source sizes of typically some 10 to some 100 micrometers.
In order to optimally align the X-ray optics with the source, very narrow tolerances must be observed. For the above reasons, misalignments of a few micrometers and some thousands of angular degrees already produce significant reflectivity and intensity losses. Temperature changes of a few degrees can cause misalignment of the optics through thermal expansion. Even a change in tube power can cause misalignment of the optics due to the associated temperature change and the associated position change of the tube focus. The optics must be adjusted relative to the X-ray source, and the beam must also be oriented towards the sample to be measured.
This produces a large number of degrees of freedom (in dependence on the type of optics and the application) which must be taken into consideration for optimum operation with fine adjustment. When rotary anode X-ray sources are used, the tube filament is disadvantageously used up after a few months of operation and must be replaced. After such a filament change, the optics must be readjusted. Readjustment of the optics is generally also required when the X-ray tube is changed.
In conventional commercially available adjusting devices, a number of fine-pitch threaded screws are usually utilized to adjust the optics in correspondence with the required degrees of freedom.
U.S. Pat. No. 5,303,035 discloses e.g. moving an inclined surface beneath an adjustment ball using manually operated adjusting screws, and transferring the resulting position change of the ball to the optical element being adjusted.
U.S. Pat. No. 5,410,206 describes a piezoelectric drive for adjusting screws for adjusting optical elements.
In the adjusting device for X-ray mirrors disclosed in the manual “Confocal Max-Flux Optics™” of the company Osmic Inc., the X-ray mirrors to be adjusted are fixed in a holding device. The adjusting elements are at a fixed position. Each change of the relative positions requires complex adjustment of the environment or reconstruction of the optical elements. The optical elements are laterally adjusted by fine-pitch threaded drives that act on the mirror or its holder from the outside. The motion is indirectly transmitted from the outer side of the housing via rods or plungers. Depending on the construction, the adjusting elements project from the sides of the device housing. When motoric drives are used, the lateral space requirements are often considerably increased. X-ray analysis systems usually only have limited installation space. Optimum positioning is often not possible due to cost or construction considerations, such that unfavorable compromises have to be accepted.
Since the X-rays are reflected on the mirror surface only within a very small angular range, the mirrors must be very finely adjusted to both compensate for production and assembly tolerances and also for adjusting the X-rays onto the sample or the detector. Due to the narrow tolerances, mechanical adjustment is often difficult and generally time-consuming, even for experienced experts. Such adjustment is often excessively demanding for the user. Since X-ray analysis today increasingly offers so-called full protection devices for radiation protection, which do not provide adjustment of the open X-ray beam, adjustment of the optics in accordance with prior art requires an extremely demanding iterative process which comprises opening the protective housing, turning one of the adjusting screws, closing the protective housing, measuring the intensity, opening of the protective housing, turning an adjusting screw etc. for each of typically four degrees of freedom.
The above-mentioned adjusting problems limit customer acceptance, and thereby possible economic profit associated with X-ray mirrors as well as other optical X-ray analysis elements which are also difficult to adjust.
It is therefore the underlying purpose of the invention to propose a device of the above-mentioned type which has a compact design and can be flexibly used, and also considerably facilitates adjustment of the optical elements.